1. Field of the Invention
The present invention relates to improvements of a casing for storing an airbag of an airbag restraint system which casing is adapted to b e deployed under the influence of high pressure gas from an inflator which is operated in response to impact at a vehicle collision or the like.
2. Description of the Prior Art
Conventionally, a variety of casings for storing an airbag have been known, in which polyurethane foam is used as its material in order to provide a soft feeling to the skin layer part of the casing which part constitutes an interior part of a vehicle. In these casings, a hard resin material or a net-like fibrous reinforcing material is used as a core layer in the casing in order that the casing tears only along a predetermined tearing line (a thin wall part) when the airbag operates as disclosed, for example, in Japanese Patent Laid-Open No. 63-232052.
However, drawbacks have been encountered in these casings, in which the casings are of a multi-layer structure and therefore is complicated in its fabrication process thereby lowering a yield or productivity and having a tendency that an unevenness is formed at the surface owing to an insert and a core layer used. Additionally, the casing in which the fibrous reinforcing material is inserted therein has a problem that the fibrous reinforcing material tends to shift in location during fabrication because of a low rigidity of the fibrous reinforcing material.
From viewpoints of solving this problem and lowering a production cost, it has been proposed to produce the airbag storing casing of a thermoplastic elastomer by an injection molding without using the above-mentioned insert, as disclosed, for example, in Japanese Patent Laid-Open No. 2-171364.
However, the thus produced casing has a thin wall section at its tearing portion of a lid member. Besides, the casing is in a structure in which the wall thickness continuously increases form the thin wall section of the tearing portion, so that the area of a thin wall is large.
As a result, this thin wall section tends to be affected by light and heat inside the vehicle, and therefore the physical properties and appearance of the casing is deteriorated with time.
In view of these problems, it will be supposed to form a decorative protecting layer or coating film at the surface of the casing. However, in case of using a polyolefin material as the material of the casing for storing an airbag, a so-called two-coat and two-bake manner or a so-called two-coat and one-bake manner are necessary to rigidly bond the coating film to the surface of the casing, thus increasing a production cost. In the two-coat and two-bake manner, a primer is first coated and dried, and thereafter a paint is coated and dried. In the two-coat and one-bake manner, a primer is coated, and then a paint is coated and dried.
In order to solve the above-discussed difficulties encountered in painting processes, the present inventors have proposed new materials for the airbag storing casing and for the coating film, suitable for a so-called one-coat and one-bake manner in which a paint is coated and dried, as disclosed in Japanese Patent Laid-Open No. 6-156178 and Japanese Patent Laid-Open No. 8-324371. In this type of airbag storing casings, as shown in FIG. 1, the airbag storing casing 1 has a coating 3 which is applied onto the main body 5 and the coating can inhibit the thin wall portion 7 at which the casing 1 is torn from being deteriorated with time. According to these proposals, an airbag storing casing which has good appearance and which can be torn surely even in a condition of low temperatures.
However, the process for preparing the paint and forming a surface layer for decoration and protection becomes troublesome, thereby increasing a production cost. Accordingly, an effort to lower the production cost becomes necessary.